Inhibiting premature vulcanization with aminothiopyrimidines

ABSTRACT

A method is disclosed for inhibiting the premature vulcanization of rubber with compounds of the formula   WHEREIN X and X1 independently are hydrogen or lower alkyl, R is hydrogen, alkyl, aralkyl or cycloalkyl, R1 is alkyl, aralkyl or cycloalkyl or R and R1 together with the nitrogen atom form a heterocycle selected from the group consisting of morpholino, alkyl substituted morpholino and   WHEREIN A and A1 independently are hydrogen or lower alkyl and n is 2 to 6.

United States Patent [1 1 DAmico Oct. 1, 1974 1 1 INHIBITING PREMATURE VULCANIZATION WITH AMINOTHIOPYRIMIDINES [63] Continuation-impart of Ser. No. 107,560, Jan. 18,

[52] US. Cl. 260/795 B, 260/247.l, 260/251, 260/780 [51] Int. Cl. C08f 27/06, C080 11/60 [58] Field of Search 260/795 B, 251 QB, 247.1, 260/780 [56] References Cited UNITED STATES PATENTS 3,513,139 5/1970 Coran 260/795 FOREIGN PATENTS OR APPLICATIONS 795,174 5/1958 Great Britain 260/795 Primary Examiner.loseph L. Schofer Assistant Examiner-C. A. Henderson, Jr.

[5 7] ABSTRACT A method is disclosed for inhibiting the premature vulcanization of rubber with compounds of the formula wherein X and X, independently are hydrogen or lower alkyl, R is hydrogen, alkyl, aralkyl or cycloalkyl, R, is alkyl, aralkyl or cycloalkyl or R and R, together with the nitrogen atom form a heterocycle selected from the group consisting of morpholino, alkyl substituted morpholino and wherein A and A, independently are hydrogen or lower alkyl and n is 2 to 6.

6 Claims, No Drawings INHIBITING PREMATURE VULCANIZATION WITH AMINOTHIOPYRIMIDINES This application is a continuation-in-part of application Ser. No. 107,560 filed Jan. 18, 1971 now abandoned.

This invention relates to an improved process for preparing vulcanizable rubber and to the rubber stocks obtained by using the improved process. The invention relates to improved accelerator-inhibitor combinations for rubber. The invention also relates to compounds useful as inhibitors of premature vulcanization in rubber. More particularly, the invention relates to a method for preventing the premature vulcanization of rubber stocks and to the rubber stocks obtained by using this method.

BACKGROUND OF THE INVENTION Premature vulcanization has been partially controlled by delayed action accelerators of which the widely used thiazolesulfenamides are a notable example. Pyrimidinesulfenamides have been suggested as accelerators of vulcanization, British Pat. No. 795,174, May 21, 1958, but never achieved acceptance.

SUMMARY OF THE INVENTION According to this invention, it has been discovered that pyrimidinesulfenamides inhibit prevulcanization if added to a vulcanizable elastomer along with a sulfur vulcanizing agent and any of the commonly used organic accelerators. Although the property of inhibiting prevulcanization appears to be a general characteristic of pyrimidinesulfenamides, a significant feature of this invention is that the performance of delayed action thiazolesulfenamide accelerators is significantly im proved by conjoint use with a pyrimidinesulfenamide. The pyrimidinesulfenamides utilizable in the practice of the invention may be represented by the formula where X and X independently are hydrogen or lower alkyl, R is hydrogen, alkyl, aralkyl or cycloalkyl, R is alkyl, aralkyl or cycloalkyl or R and R together with the nitrogen atom form a heterocycle selected from the group consisting of morpholino, alkyl substituted morpholino and where A and A, independently are hydrogen or lower alkyl and n is 2 to 6. The alkyl radicals may be primary, secondary or tertiary and the alkyl chain may be branched or unbranched. Alkyl radicals of l to 20 carbon atoms are suitable with radicals of l to 10 carbon atoms being preferred. Lower alkyl means radicals of l to 5 carbon atoms. Cycloalkyl radicals of 5 to 12 carbon atoms are suitable with radicals of 5 to 8 carbon atoms being preferred. Aralkyl is a monovalent alkyl radical having a phenyl radical attached to a carbon atom of the alkyl chain. Aralkyl radicals of 7 to 10 carbon atoms are preferred.

' Examples of R and R, are methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl, hexyl, octyl, decyl, dodecyl, cetyl, eicosyl, cyclopentyl, cyclohexyl, cyclooctyl, cyclodecyl, cyclododecyl, benzyl, alpha-methyl benzyl, alpha, alpha-dimethyl benzyl and phenethyl. Examples of amino radicals, wherein R and R together are alkylene, are pyrrolidinyl, 2,5- dimethyl pyrrolidinyl, piperidino, Z-methylpiperidino,

2,6-dimethylpiperidino, hexamethyleniminyl(hexahydpyrimidine,

ro- 1 H-azepinl -yl), hexahydro-l (2H )-azocinl -yl and octahydrol H-azoninl -yl.

Examples of the compounds of the invention are 2- (dimethylaminothio)pyrimidine, 2-(dibutylaminothio)- Z-(dihexylaminothio)-4,6- dimethylpyrimidine, 2-(dibenzylaminothio)pyrimidine,

2-(dibenzylaminothio)-4,6-dimethylpyrimidine, 2- (dicyclopentylaminothio )pyrimidine, 2- (dicyclohexylaminothio)-4,6-dimethylpyrimidine, 2-

(dicyclooctylaminothio)pyrimidine, Z-(methylaminothio)pyrimidine, 2-(methylaminothio)-4,6- dimethylpyrimidine, Z-(ethylaminothio)pyrimidine, 2- (propylaminothio)-4,6-dimethylpyrimidine, 2-(nbutylamino)-4,6-dimethylpyrimidine, 2-(hexylaminothio)pyrimidine, 2-(octylaminothio)-4,6- dimethylpyrimidine, 2-(cyclohexylaminothio)- pyrimidine, 2-(cyclohexylaminothio)-4- methylpyrimidine, 2-(tert-butylaminothio)pyrimidine, 2-(isopropylaminothio)-pyrimidine, 2- (cyclododecylaminothio)-4,6-dimethylpyrimidine, 2-(morpholinothio)pyrimidine, 2-(2,6-dimethylmorpholino)-pyrimidine, 2-( 1-pyrrolidinylthio)pyrimidine, 2-(2,5-dimethyl-1-pyrrolidinylthio)pyrimidine, 2-(piperidinothio)pyrimidine, 2-(2-methylpiperidinothio)pyrimidine, 2-(hexahydrol (2H)-azocinl ylthio)pyrimidine, 2-(octahydrol H-azoninl -ylthio)- pyrimidine, 2-(pyrrolidinylthio)-4,6- dimethylpyrimidine, 2-(2,5-dimethylpyrrolidinylthio)- 4,6-dimethylpyrimidine, 2-(piperidinothio)-4,6- dimethylpyrimidine, 2-hexahydro- 1 (2H )-azocinl ylthio)-4,6-dimethylpyrimidine, 2-(octahy'dro-1H- azonin-l-ylthio)-4,6-dimethylpyrimidine and i 2- (hexahydrol H-azepinl -ylthio )pyrimidine.

The compounds of this invention are excellent prevulcanization inhibitors and secondary accelerators which when added to sulfur-vulcanizable compositions provide stocks having greater processing safety which cure faster. Both properties are advantageous to the rubber manufacturer since greater processing safety eliminates wasted stocks ruined by premature vulcanization and faster cure rates increase productivity by decreasing the time an article remains in the vulcanization mold.

The inhibitors of the invention may be used in natural and synthetic rubbers or mixtures thereof. Any diene rubber having sufficient unsaturation to be sulfur vulcanizable is suitable. Examples of suitable synthetic rubbers are styrenebutadiene copolymer (SBR), isobutylene-isoprene copolymer (butyl), ethylene-propylene diene terpolymers (EPDM), butadieneacrylonitrile copolymer (nitrile), polymers of chloroprene, neoprene and synthetic polybutadiene, particularly cispolyisoprene and cis-polybutadiene.

The advantages of the invention are obtained in vulcanizable compositions containing vulcanization accelerators of various classes and mixtures thereof. For example. rubber mixes containing the aromatic thiazole accelerators which include N-cyclohexyl-2-benzothiazolesulfenamide, N,N-dialkyl-2-benzothiazolesulfenamide, 2-mercaptobenzothiazole, N-tert-butyl-Z- benzothiazolesulfenamide, 2-benzothiazolediethyldithiocarbamate, 2,2-dithiobisbenzothiazole and 2- morpholino-thiobenzothiazole may be used. The amine salts of mercaptobenzothiazole accelerators, for examplc the tert-butylamine salt, morpholine salt or 2,6-dimethylmorpholine salt of mercaptobenzothiazole, may be used in the invention. Thiazole accelerators other than aromatic can be used. Stocks containing other accelerators, for example dithiocarbamate accelerators, tetramethylthiuram disulfide, tetramethylthiuram monosulfide and zinc dibutyldithiocarbamate or aldehyde amine condensation products and diary] guanidine derivatives are substantially improved. The invention is applicable to rubber mixes containing sulfur-vulcanizing agents. The amount used generally is between 0.2-3.0 parts per hundred parts rubber. For the purpose of this invention sulfurvulcanizing agent means elemental sulfur or sulfurcontaining vulcanizing agent, for example, amine disulfide or polymeric polysulfide. Rubber stocks containing antidegradants, for example, N-1,3-dimethylbutyl-N- phenyl-p-phenylenediamine, N,N '-bis- 1 ,4- dimethylpentyl-p-phenylenediamine, N,N'-bis-2-octylp-phenylenediamine and other phenylenediamines or mixtures thereof and ketone, ether and hydroxy antidegradants and mixtures thereof are substantially improved by the process of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inhibitors of the invention are conveniently prepared at moderate temperatures by the oxidative condensation of the appropriate amine with a pyrimidinethiol. The products are recovered and purified by conventional procedures.

EXAMPLE 1 2-(Morpholinothio)-4,6-dimethylpyrimidine is prepared in the following manner: 44.2 grams (0.25 mole) of 4,6dimethyI-Z-pyrimidinethiol hydrogen chloride (J. Chem. Soc. 1959, p. 525), 40 grams (0.25 mole) of 25 percent sodium hydroxide solution and 32.6 grams (0.375 mole) of morpholine are dissolved in 400 ml of isopropyl alcohol. The mixture is heated with stirring for one hour at 45-50C., 197 ml (0.375 mole) of (14.28 g/ 100 ml) sodium hypochlorite solution is added dropwise under the surface over a two hour period at the temperature of 4550C. The mixture is stirred at 4550C for one additional hour. The excess hypochlorite is destroyed by the addition of 5 grams of sodium sulfite dissolved in one liter of water. The product is cooled to 5C, separated by filtration, washed with water until neutral and air dried at 2530C. 24 Grams (43 percent yield) of the desired product, m.p. 99100C, is obtained. After recrystallization from ethyl alcohol, the melting point remained unchanged. Analysis gives 18.51 percent nitrogen and 14.09 percent sulfur compared to 18.65 percent nitrogen and 14.23 percent sulfur calculated for C H N OS.

EXAMPLE 2 2-(2,6-Dimethylmorpholinothio)-4,6-

dimethylpyrimidine is prepared by a procedure similar to Example 1 except 2,6-dimethyl morpholine is used in place of morpholine. 35 Grams (55 percent yield) of the desired product, a water insoluble solid is obtained. Recrystallized from heptane, the product melts at 73-74C. Analysis gives 16.03 percent nitrogen and 12.79 percent sulfur compared to 16.59 percent nitrogen and 12.66 percent sulfur calculated for C H N OS.

EXAMPLE 3 EXAMPLE 4 2-(Cyclohexylaminothio)-4,6-dimethylpyrimidine is prepared in a manner analogous to Example 3 except the amine used is cyclohexylamine. 29 Grams (45 percent yield) of product is obtained. Recrystallized from heptane, it melts at 72-73C. Analysis gives 60.89 percent carbon, 7.97 percent hydrogen, 17.58 percent nitrogen, 13.32 percent sulfur compared to 60.72 percent carbon, 8.07 percent hydrogen, 17.70 percent nitrogen, 13.51 percent sulfur calculated for C H N s.

EXAMPLE 5 2-(tert-Butylaminothio)-4,6-dimethylpyrimidine is prepared in the following manner: 44.2 Grams (0.25 mole) of 4,6-dimethy1-2-pyrimidinethiol hydrogen chloride, 40 grams (0.25 mole) of 25 percent sodium hydroxide solution and 73.2 grams (1.0 mole) tertbutylamine are slurried in 300 ml of isopropyl alcohol. The slurry is heated with stirring for. 30 minutes at 45--50C, 170.8 ml (0.375 mole) of (16.4 g/l00 ml) sodium hypochlorite solution is added dropwise over a two hour period at the temperature of 45-50C. Two

liquid phases are observed. After cooling to 30C, the excess hypochlorite is destroyed by adding 5 grams of sodium sulfite. 200 Grams of sodium chloride are added and the mixture stirred for thirty minutes. The salt is filtered from the solution. The aqueous layer of the filtrate is separated. The organic layer is concentrated to about ml in vacuo. 300 ml of ethyl ether is added to the concentrate and stirred for 5 minutes.

The ether layer is separated and is dried with anhydrous sodium sulfate. The ether is removed in vacuum at a maximum temperature of 30C at l-2 mm Hg pressure. 29 Grams (55 percent yield) of an amber liquid is obtained. Analysis gives 56.80 percent carbon, 8.17 percent hydrogen, 19.73'percent nitrogen, 15.08 percent sulfur compared to 56.83 percent carbon, 8.11

percent hydrogen, 19.89 percent nitrogen, 15.17 percent sulfur calculated for c,.,H,,N s. The molecular weight determined in acetone is 215 versus the theoretical value of 211.3.

EXAMPLE 6 6 The time t in minutes required for the moiie 'ifiin to rise 5 points above the minimum viscosity is recorded. Longer times on the Mooney scorch test are indicative of greater processing safety. The cure characteristics of the stocks are determined by a Monsanto Oscillating Disk Rheometer at 292F. The time in minutes required for a rise of 2 rheometer'units above the minimum reading and the time required to ob tain 90 percent of the rheometer maximum torque are recorded. The difference, t t is indicative of the cure rate of the stocks. Small values of t -t indicate faster cure rates. The vulcanization rate K is measured in reciprocal minutes. See Coran, 37 Rubber Chemistry and Technology 689 (1964). Higher values of K indicate higher rates of cross link formation. Vulcanizates are prepared by press curing the stocks at 292F for the time required to achieve optimum cure as indicated from the rheometer data and the physical properties of the vulcanizate are measured and recorded.

TABL l Stock No.

Natural Rubber Smoked sheets 1SAF Carbon Black Zinc Oxide Stearic Acid Hydrocarbon Softener N-( l,3-Dimethylbutyl)-N'- phenyl-p-phenylenediamine Sulfur N-rerl-Butyl-2-benzothiazolesulfenamide 2-(lert-Butylaminothio)-4,6- dimethylgyrimidine Z-(Cyc o exy1aminothio)-4.6- dimethylpyrimidine 2-(|sopropy1aminothio)-4.6- dimethylpyrimidine Mooney Data at 250F t minutes Increase in Scorch Delay Rheometer Data at 292F t minutes Maximum torque A PP Mww UIO 00000 c Stress-Strain Data at 292F Cure Time, minutes 300% Modulus, psi Ultimate Tensile Strength, psi

Ultimate Elongation,

Stock No.

Natural rubber smoked sheets ISAF Carbon Black Zinc Oxide Stearic Acid Hydrocarbon Softener Sulfur phenyl-p-phenylenediamine 5 2 N-( l,3-Dimethy1buty1)-N'- 2.

N-rert-Butyl-2-benzothia' 0 zolesulfenamide thio)-4,6-dimethy1pyrimidine 2-( Morpholinothio )-4,6-dimethylpyrimidine 2 (Hexahydro-1 H-azepin- 1 ylthio)-4.6-dimethylpyrimidine Mooney Data at 250F t minutes Increase in Scorch Delay 7 TABLE lI-Continued Stock No. l 2 3 4 Rheometer Data at 292F t minutes 7.9 8.3 8.4 7.8 1, minutes 20.2 17.3 17.0 15.2 h -t 12.3 9.0 8.6 7.4 Maximum torque 53.6 61.0 62.3 65.0 Minimum torque 4.2 3.7 3.8 4.0

The data show the inhibitors derived from primary 10 CH u amines are more active than those derived from hetero X-o o-x cyclic amines and that in addition to increasing processing safety, the prevulcanization inhibitors also activate the cure. Results comparable to those in the above table are obtained with other compounds of the invenl5 tion and also in stocks prepared from styrene- R! butadiene rubber.

Although the invention has been illustrated by typical examples, it is not limited thereto. Changes and modifications of examples herein chosen for purposes of dis- 20 closure can PF made which do P consfltute departure wherein X and X independently are hydrogen orlower from the and Scope 9 the alkyl, R is hydrogen R, is alkyl of l to carbon atoms, The embodiment of the ll'lVel'ltlOn m Whlch an exclu' aralkyl of 7 to 10 carbon atoms and cycloalkyl of 5 to sive property or priv lege is claimed are defined as fol- 12 carbon atoms Y I b] If 2. A composition of claim 1 wherein the accelerating e composl compnslpg a agent is a benzothiazolesulfenamide. vulcamzable diene rubber, a sulfur vulcarnzmg agent, 3 A composition of claim 1 wherein X and X are an accelerating agent selected from the group consistmethyling of thiazole accelerators, dithiocarbamate accelera- A composition of Claim 3 wherein R is hydrogen tors, thiuram sulfide accelerators, aldehyde-am ne acand R1 is lower alkyL celerators, dlaryl guamdme accelerators and m1xtures A composition of claim 3 wherein R is hydrogen thereof in an amount effective to accelerate vulcanizaand R is c l h 1 1 yo 0 exy tion and an amount effective to inhibit premature vul- A composition of Claim 4 wherein R1 is terpbuty]. canization a compound of the formula 

1. A VULCANIZABLE COMPOSITION COMPRISING A SULFUR VULCANIZABLE DIENE RUBBER, A SULFUR VULCANIZING AGENT, AN ACCELERATING AGENT SELECTED FROM THE GROUP CONSISTING OF THIAZOLE ACCELERATORS, DITHIOCARBAMATE ACCELERATORS, THIURAM SULFIDE ACCELERATORS, ALDEHYDE-AMINE ACCELERATORE, DIARYL GUANIDINE ACCELERATORS AND MIXTURES THEREOF IN AN AMOUNT EFFECT TO ACCELERATE VULCANIZATION AND AN AMOUNT EFFECTIVE TO INHIBIIT PREMATURE VULCANIZATION A COMPOUND OF THE FORMULA
 2. A composition of claim 1 wherein the accelerating agent is a benzothiazolesulfenamide.
 3. A composition of claim 1 wherein X and X1 are methyl.
 4. A composition of claim 3 wherein R is hydrogen and R1 is lower alkyl.
 5. A composition of claim 3 wherein R is hydrogen and R1 is cyclohexyl.
 6. A composition of claim 4 wherein R1 is tert-butyl. 